1. Field of the Invention
The present invention relates to a charge transfer device, and more particularly to power saving in an output section of a charge transfer device.
2. Description of the Related Art
FIG. 1 is a schematic circuit diagram illustrating the structure of an output section of a solid-state imaging device using a CCD image sensor. Information charges corresponding to each pixel generated in an imaging section 2 of the CCD image sensor are sequentially transferred via a horizontal CCD register 4 to a floating diffusion layer 6.
The information charges are then converted into an image signal, which is a voltage signal and which is supplied, as an input, to an output amplifier 8. The output amplifier 8 is a source follower amplifier circuit in which a transistor is used for the load of a source and which, in this case, includes interconnected three-stage source followers. Each source follower circuit is connected between a power source VD and ground GND. An output signal from the floating diffusing layer 6, or a source follower output at the previous stage, is input to a gate of each of transistors TD1, TD2, TD3 for amplification. To a source of each of the transistors TD1, TD2, TD3, a drain of each of load transistors TL1, TL2, TL3 is connected. These load transistors function as a constant current source for supplying a constant current between the source and the drain in accordance with a prescribed gate bias voltage.
A CCD output obtained from the source follower at the final stage as an output signal of the CCD image sensor is input into a peripheral circuit 10. In this peripheral circuit 10, the CCD output is received by an emitter follower amplifier circuit 12, and the amplified signal is then input to a front end circuit 14.
The conventional output amplifiers are designed such that a sufficient band can be obtained considering the frequency characteristics of an image signal obtained at the time of normal imaging operations. From the same point of view, the gate bias voltage is set to a predetermined value. Although, in order to improve the frequency characteristics, it is generally effective to increase current flowing in the source follower, this creates a disadvantage that power consumption in the source follower is increased. This disadvantage also appears in the emitter follower amplifier circuit, which is a peripheral circuit. More specifically, power consumption is increased because the current flowing in the emitter follower amplifier circuit must be increased as necessary to prevent deterioration of the frequency characteristics of a CCD output.
Further, because the output amplifier is continuously operating even when an image signal is not being generated, extraneous signals other than the image signal is also amplified in the output amplifier or the like, and is then input to an image signal processing circuit at the subsequent stage.